1. Technical Field
The present disclosure relates generally to the logging of subsurface formations surrounding a wellbore using a downhole logging tool, and particularly to using the logs to estimate one or more formation properties.
2. Background Art
Logging tools have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Common logging tools include electromagnetic tools, nuclear tools, and nuclear magnetic resonance (NMR) tools, though various other tool types are also used.
Early logging tools were run into a wellbore on a wireline cable, after the wellbore had been drilled. Modern versions of such wireline tools are still used extensively. However, the need for information while drilling the borehole gave rise to measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools. MWD tools typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools typically provide formation evaluation measurements such as resistivity, porosity, and NMR distributions (e.g., T1 and T2). MWD and LWD tools often have components common to wireline tools (e.g., transmitting and receiving antennas), but MWD and LWD tools must be constructed to not only endure but to operate in the harsh environment of drilling.
Determining fluid types from continuous nuclear magnetic resonance multi-dimensional logging can be accomplished in certain situations, such as where the signal-to-noise ratio is relatively high. However, this technique has not been possible to be applied to tight shale gas formations because of the low signal-to-noise ratio associated with such formations. For example, the free fluid volume in shale is typically small and the gas therein may not completely polarize. Furthermore, there is a low hydrogen index. Those factors contribute to a weak measured signal while the noise level is generally high because of high formation temperatures, thereby making the signal-to-noise ratio unfavorable to continuous nuclear magnetic resonance logging.
A logging tool such as Schlumberger's ECOSCOPE tool is capable of measuring multiple depths of investigation ranging from about 6 inches to 12 inches into the formation. There is a need to determine porosity and individual fluid volumes and saturations such as adsorbed and free gas, oil, oil-based mud filtrate, and bound and free water in low porosity shale gas formations from NMR multi-dimensional logging data. There is similarly a need to determine the kerogen volume in the formation.